The P300 or P3, a late positive component of sensory evoked potentials, is said to reflect a variety of cognitive functions and to provide an objective index of information processing in normal and neurologically compromised individuals. An important issue in cognitive neuroscience has been the identification of the brain structures that generate it. However, extensive investigations conducted for that purpose and involving intracranial recordings and mapping of its scalp distribution have been inconclusive. Using magnetoencephalography (MEG) a method akin to traditional electrophysiology, but of greater spatial resolution we have recorded the magnetic equivalent of P3 to auditory stimuli and have localized its sources on the superior surface of the temporal lobe. We also found evidence that deeper sources (possibly thalamic) may contribute to its onset. These results lead to two general hypotheses: (a) That P3s are modality-specific and (b) that activation of modality-specific cortical regions is preceded by early thalamic activity. We propose to test these general hypotheses in a series of twelve experiments where P3s to rare and to unexpectedly missing auditory, visual and somatosensory stimuli will be obtained under two conditions. One will involve attention to stimuli resulting in a long latency P3, which is thought to reflect mostly higher order cognitive operations. The other will involve ignoring the stimuli, resulting in a shorter latency P3 associated with more basic sensory functions. The estimated sources of P3s obtained with the different stimulus modalities and conditions, projected onto individual magnetic resonance images (MRIs) will be used to answer whether P3s are modality-specific, whether they share an early thalamic origin and whether different structures generate P3s to the presence or absence of attended or incidentally registered unexpected environmental events.